The present invention relates to slip and snubbing assemblies and, more particularly, to a slip spool used to selectively support or snub a tubing string during a well operation.
In the oil industry, slips have been essential components of oil field drilling and servicing equipment for many years. Conventional slips are sets of heavy hinged blocks with gripping dies that are positioned in a slip bowl of a rotary table to engage a drill pipe, casing or production tubing. Angled surfaces in each slip block mate with angled surfaces in the slip bowl. The angled surfaces cause axial forces exerted by the weight of the pipe on the blocks to be transferred into lateral gripping pressure on the pipe, which supports the pipe and thus prevents it from dropping into the bore hole.
As is well known in the art, conventional slips are manually engaged by oil field personnel who physically maneuver the slips into the slip bowl so that they slide into engagement with the casing or drill pipe. The slips are disengaged by upward axial movement of the casing, drill pipe, or production tubing to take the weight off the slips. The slips are then lifted out of the slip bowl. An example of such conventional slips is described in U.S. Pat. No. 4,244,093, which is entitled TUBING SLIP PULLING TOOL and issued to Klingensmith on Jan. 13, 1981.
As is also well known in the art, certain wells have natural pressure that may overburden the weight of a plugged tubing string. Consequently, maneuvering the tubing string requires a snubbing unit to prevent the tubing string from being ejected from the well by the natural well pressure. The snubbing unit functions like invented slips, and grips the tubing string to hold it down against the fluid pressure in the well.
There is an ever increasing demand for obtaining more oil and gas from existing wells. After a primary recovery term of a well has expired, some form of reworking is required to obtain remaining oil and/or gas from the well. Usually in reworking those wells, such as in preparation for a well stimulation process, the tubing string must be removed from the well or pulled up for attachment of wellhead tools, and then lowered again to insert the wellhead tools through the wellhead. During such operations, the tubing string is secured by slips or, less often, by a snubbing unit. It is therefore necessary to remove and set the slips or the snubbing unit in preparation for a well stimulation process. Consequently, slips and snubbing units are not only frequently used during well drilling and completion, they are also required equipment for well re-completion, servicing and workover.
However, the handling of slips and snubbing units can be dangerous and time-consuming. Mechanical equipment for moving slips has also been utilized in the past to alleviate the manual labor. An example of a hydraulically operated slip assembly used to grip pipe as it is being run into or pulled from a well is described in U.S. Pat. No. 5,027,926, which is entitled SLIP ASSEMBLY and issued to Cox on Jul. 2, 1991. Although Cox eliminates the manual handling of slips, he does not address the problem of pressure containment, which becomes an issue when a live well requires some form of rework or stimulation. Without pressure containment, a live well must be xe2x80x9ckilledxe2x80x9d before control valves or a BOP in the wellhead can be opened. As is known in the art, killing the well is a time-consuming and expensive process. Killing the well may also undo a significant port of the benefits of well stimulation.
The problem of manipulating tubing in a live well has also been addressed, however. Light-duty slips for use with coiled tubing have been invented, as described in U.S. Pat. No. 5,590,867, entitled BLOWOUT PREVENTER FOR COILED TUBING, which issued to Van Winkle on Jan. 7, 1997. The slips incorporated in Van Winkle""s blowout preventer do not have a conventional wedge structure, however. Consequently, the gripping pressure on the pipe is not increased by the weight of the pipe, and the slips incorporated in Van Winkle""s blowout preventer are not adapted to support the considerable weight of a jointed tubing string.
Another disadvantage of prior art slip assemblies arises because the tubing string is supported by friction forces between the slips and the exterior surface of the tubing under the gripping pressure on the tubing exerted by the slips. In order to increase the friction, the gripping surface of the slips is usually provided with gripping teeth. Although ideally slips do not damage the tubing surface, it has been found that even a single actuation of the slips against the tubing can score the exterior surface of the tubing. In today""s high performance well operations at elevated fluid pressures, this scoring can reduce the useful life of the tubing, particularly in the case of coil tubing, and potentially cause fluid leaks as the tubing is pulled through seals, such as the tubing rams of a BOP.
There is therefore a need for a pressure containing slip spool that overcomes the shortcomings of prior art slip assemblies.
An object of the invention is to provide a pressure containment slip spool for selectively supporting a tubing string suspended in a wellbore, by engaging a component in the tubing string having a weight-bearing shoulder adapted to be engaged by slip members of the slip spool.
A further object of the invention is to provide an apparatus for selectively supporting a tubing string in a wellbore, which does not score an exterior surface of the tubing and can be used for either slip or snub operations.
The invention therefore, provides an apparatus that includes a slip spool that can be mounted to a wellhead for selectively supporting a tubing string suspended in the wellbore. The slip spool has an axial passage to be aligned with the wellbore for permitting the tubing string to extend therethrough, and at least two radial passages extending through a wall of the slip spool and communicating with the axial passage. The slip spool is provided with at least two slip blocks which are slidably supported within the respective at least two radial passages. The slip spool further includes means for moving the respective slip blocks between an extended position in which the slip blocks engage a component in the tubing string having a weight-bearing shoulder adapted to be engaged by the slip blocks, and a retracted position in which the slip blocks clear the axial passage of the slip spool.
In accordance with another aspect, the invention provides an adapter pin for providing the weight-bearing shoulder in the tubing string. The adapter pin comprises a tubular collar having threads at a top end and at a bottom end thereof for connecting the adapter pin to a tubular, tubing head or a downhole tool. The bottom end preferably further includes an inwardly and upwardly bevel that functions as the weight-bearing shoulder. The slip blocks in the slip spool are adapted to engage the weight-bearing shoulder of the adapter pin, thereby supporting the weight of the tubing string.
In an alternate embodiment, the adapter pin is configured to function for both slip and snub operations. The embodiment of the adapter pin includes an area of reduced diameter located between the top and bottom ends; a top edge of the area provides the weight-bearing shoulder for slip operations and a bottom edge of the area provides the weight-bearing shoulder for snubbing operations. The weight-bearing shoulders may be inwardly beveled, and the slip blocks may be ridged to engage the bevels, to ensure that the slip blocks cannot disengage the adapter pin unless a load extended by the tubing string is removed from the slip blocks.
The apparatus in accordance with the invention permits the slip blocks to be extended and retracted in a convenient and safe manner, particularly during a live well operation. Scoring of the exterior surface of the tubing is also eliminated, thereby increasing the life expectancy of the tubing.
Other advantages and features of the invention will be better understood with reference to preferred embodiments of the invention described hereinafter.